1. Field of the Invention
The invention relates to a method for laser marking security documents and a corresponding device which enable the storage of graphic information, which describes a graphic having different brightness values, in a security document.
2. Brief Description of the Related Art
From prior art, it is known to mark security documents using laser radiation. In particular security documents, which are produced based on plastics, can be marked by means of laser radiation such that graphic information is stored on the surface of or inside the security document. To achieve storage inside a security document, pigments are for example added to a transparent material layer, which indeed promote absorption of laser radiation but do not at all or only insignificantly impair a general transparency of the material. The laser irradiation is absorbed locally in the layer provided with the pigments and this, for example, causes a carbonation of the plastic. Depending on the energy irradiated, different degrees of blackening, i.e. different grey scales, appear. The energy necessary for generating a certain grey scale is thus dependent on the security document to be marked.
For identically-made security document blanks, the energy quantity needed to induce a certain degree of blackening or a certain grey scale value is already known. In devices known from prior art, lasers are used which can be controlled, as regards their laser light output or a laser pulse energy, via a laser control signal. Preferably, pulsed lasers are used for laser marking, and the graphic information is composed of dots. Herein, all graphically representable information is viewed as graphic information. Thus, the graphic information can comprise an image, e.g. a facial image, a line drawing, text, numbers or similar, individually or in any combination. Particularly preferably, exactly one laser pulse is used for marking for each pixel which has a certain grey scale value. However, it is also possible to use several laser pulses of defined energy for generating one and the same pixel or to partially overlap several dots. However, the laser output or laser pulse energy can only be influenced relatively by such a laser control signal. This means that an accurate laser output or laser pulse energy cannot be accurately specified. Therefore, it is necessary to perform a calibration in the manner that, for each grey scale value which is to be marked, a calibration is performed in the manner that the laser control signal, i.e. the control value of the laser control signal, is varied until a marking with the corresponding grey scale value is achieved. Using the control values, thus ascertained, of the laser control signal, an assignment function can then be generated which assigns a corresponding control signal value to grey scale values to be marked, so that a corresponding laser pulse energy or laser pulse output for marking a pixel with the corresponding grey scale value is achieved. Since, due to thermal fluctuations and other environmental influences for the same control value of the laser control signal, the actually generated laser light output or laser pulse energy can vary slightly, it is known from prior art to decouple a part of the generated laser light onto a photodiode and to use the signal thereof, which is a measure for the captured laser output or pulse energy of the decoupled laser light, as a control signal for a control circuit, said control circuit effecting a correction of the laser control signal. While this makes it possible to use a calibrated device for laser marking over longer periods in order to obtain security documents having sufficient quality and reproducibility of the individual grey scale values, recalibration cycles are necessary at intervals, in which the actually generated grey scale values are compared to the grey scale values to be achieved. These calibrations can only be performed by specially trained technicians and/or require sophisticated optical measuring devices which can accurately compare the grey scale values to each other.
It should be noted here that the optically perceived grey scale value is usually determined by a plurality of pixels. On the one hand, the actual grey scale value of the individual pixels plays a role. Likewise, however, the perceivable grey scale value is also affected by a pixel density, i.e. a number of pixels per area. Usually, the individual pixels are each introduced into a document in a grid which is uniform for a graphic information. Of course, it is also possible to arrange the pixels at different distances to each other. When ascertaining or defining the grey scale values for the individual pixels for laser marking, the distances are known at which the pixels are formed in a document. The grey scale values for the individual pixels are defined accordingly. In the security document, in which the formed pixels can partially overlap each other, a graphic information then emerges which, for an observer, produces the desired grey scale impressions. However this assumes that not only the distances of the pixels but likewise the individual grey scale values of the individual pixels, which are formed by means of laser marking, are correct.